ISSN 1009- 3095 Journal of Zhejiang University(SCIENCE) V. 2, No. 1, P. 41 - 45, Jan. - Mar., 2001
http://www, chinainfo, gov. cn/periodical; http://www, zju. edu. cn;
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41
SHORT CIRCUIT CURRENT LIMITER IN AC NETWORK
WU Zhao-lin( 5~v~gN ), CHEN P i n g - p i n g ( i ~ , ~ ), TAN Ling-yun ( i ~
), JIANG Dao-zhuo( ~32jgr
)
(State Key Laboratory of Power Electronics, Zhefiang University, Hangzhou 310027, China)
Received May. 25, 2000; revision accepted Sept. 8, 2000
Abstract:
Short circuit is a serious fault in power network. Some novel circuit topologies of current limiter
using power e]ectronie technology have been developed, which can limit the fault eun~ent to any desired level
without much penalty. The operating principle and control strategies of such current limiters are discussed in
detail. Sinmlation and expel-imental results are given to verify the performance of the current limiter, which
can meet the requirements set for locations of bus tie, feeder, as well as the main transformer in the distribution network.
Key words:
short circuit protection, current limiter, solid state switehgear, solid state current limiter
Document code:
A
CLC number:
TM91
INTRODUCTION
Short circuit in power systems is a serious
fault, which may damage the equipment in the
system . It requires the protection system to
switch off the very high level of fault current.
The traditional method to limit the short circuit
current level is to serially connect a limiting reactor in the system. However the consideration
of voltage drop and power dissipation of the reactor in no~nal operation limits the effect of the reactor. Some new approaches for short circuit
limiting had been proposed. The EPRI (Electrical Power Research Institute) in the USA carried
out a series of investigations into the technologies, which may possibly be used as short circuit
limiter. The results of the investigation (Slade et
a l . , 1992; Smith et a l . , 1993) proposed that a
limiter consisting of solid state device such as
GTO ( i n Fig. 1 ) may be the best approach
among other possible approaches . The reactor in
Fig. 1 is bypassed by the GTOs in normal operation. If short circuit occurs the GTOs should be
switched off very quickly before the short circuit
current rises to a high level. It requires fast response of the protection system. When the GTOs
are switched off', the reactor L is inserted into
the fault circuit. The interrupted current in GTO
will transfer to the reactor L with very large d i /
d t . A high frequency resonance will be excited
which causes large over-voltage with very rapid
dv/dt applied to the GTOs as well as other
equipment in the system. To reduce the negative
effect , the value of reactor used has to be limited. A traditional breaker S is also needed to finally switch off the limited short circuit current.
~AGTQ
I ,,%1
Fig. 1
Omrent limiter with GTO
Another current limiting approach, the so
called lossless resistor ( L L R ) was proposed
(Chen et al. , 1994; Chen et al. , 1 9 9 7 ) , which
uses IGBTs to form a bridge circuit and PWM
technique in power electronics ( F i g . 2 ) . According to the authors, if the bridge is operating
as rectifier and inverter alternately with modulating frequency higher than the mains frequency,
the equivalent resistance of I J,R can be controlled. The higher the modulating frequency is,
the higher is the value of LLR. With much higher modulating frequency at short circuit, the
equivalent value of LLR could be large enough to
limit the, short circuit current to a desired level.
Project(59777022) suppoded by Nation~ Natural Science Foundation of China(NSFC)
42
WU Zhaolin, TAN Lingyun et al.
With such a topology, the modulated current in
the positive half cycle is the same as that in the
negative haft cycle. The simple topology could
not operate properly. Furthermore the switch devices in the bridge are operating in hard switching condition with relatively high modulating fiequency and the switching loss is considerable
even in normal eondition. Abundant harmonics
also exist with the approach.
id
D
Load [ VLo.d
T
Fig. 3
Current limiter in DC
A novel approach for current limiting was
developed recently using power electronics technology, which can limit the current to any desired level without much penalty.
In engineering practice, the current source
ic in Fig. 3 can be replaced by a reactor L. In
this case the reactor, as well as the load, will be
charged to its steady state through initial transients. In short circuit the current is limited by
the reactor to rise slowly, which allows the protection system to respond easily and switch off
the relatively low short circuit current. The reactor current will pass through the paralleled diode
( a s a freewheeling diode ) after switching off
without any transient overvohage until the energy
stored in the reactor is exhausted.
CURRENT LIMITER IN DC CIRCUIT
CURRENT LIMITER IN AC CIRCUIT
Fig. 3 shows the principle of the proposed
current limiter. If the current of the current
source ic in normal condition keeps larger than
the value of load current i , the diode D will always be at on state. Compared with the high
voltage of the voltage source, the very low on
state voltage drop of D will not affect the load
voltage. In the case of short circuit, the source
voltage acts on the diode D reversely and turns it
off. The fault current rises instantaneously to and
is limited by the current of the current source i c .
Once the value of i,, is set, the peak value of the
short circuit current will be limited automatically
to the value of i0, which is easily switched off by
the switch S.
The idea of DC current limiter can be extended to AC network as shown in Fig. 4 ( W u ,
1 9 9 6 ; W u et a l . , 1 9 9 9 ) . In F i g . 4 b ) with La,
Lz to replace the current source in F i g . 4 a ) , the
currents of Ll and L2 will reach the peak value of
the sinusoidal load current after the initial stage
transients have receded, ff the power loss of the
reactor and diode is negligible compared to the
energy stored in the reactor, the current of each
reactor keeps constant and the current in each diode always flows except during the peak of the
load current (Fig. 4 c ) . Therefore the load current
is theoretically sinusoidal and without distortion.
If a short circuit fault occurs at any instant in a
v~n
Current Umiter (Chen et a l . , 1994)
Fig. 2
i,,
L:
[~in
Load [ ff.....
Load I k;.....
[Gn
0t V iLOad
V
Time/(1 (hns/div)
(a)
Co)
(c)
Fig.4 Current limiter in AC
(a) current limiter with current source; (b) current limiter with reactom; (c) current waveform
Short circuit current limiter in AC network
period, one of the diodes must be reversely biased and the fault current will jump at normal
load current amplitude to one of the inductors,
then rises slowly due to the limiting function of
the inductor which is automatically inserted into
the fault circuit. If a pair of parallel reversely
connected thyristors is used as S shown in Fig.
4, the fault current will be switched off automatically at the first point of source voltage across
zero without transient. The longest fault current
interruption time is one half cycle. Then the current in each reactor keeps flowing through its
freewheeling diode. Tbe reactance of the reactors
L1, I~ can be designed such that the short circuit
current should be limited to the desired level.
TOPOLOGY INNOVATION OF CURRENT LIMITER IN AC CIRCUIT
The current limiter topology (Fig. 4 b ) used
in AC circuit can be converted to a bridge topoiogy shown in Fig. 5, in which only one reactor is
t
Q~/)
~lTT'
n 1
D~
iD I
43
needed. In steady state the current in reactor L
is almost equal to the amplitude of the sinusoidal
load current. The two diodes function as freewheeling diodes and are always at on state (Fig.
5 b ) . With the two diodes at on state, the limiter has no obvious effect on the load current. If
short circuit occurs, the fault current rises instantaneously to the value of the reactor current
and one of the two diodes is reversely biased,
The reactor L is inserted into the fault circuit automatically to limit the short circuit current. If
the gate pulses are blocked in time the conducting thyristor will be turned off at the next point of
source voltage across zero (Fig. 5 e ) . The current in the reactor is then freewheeled through
the two diodes. At short circuit, the fault current can be calculated as follows:
"."
dis
vin=Ldt
is=10+Ai
.'.
Ai = ~1
Vr~ ( 1 + eoscot0 )
2%sincotdcot = coL
lLold
O[ U U ~
Time(l Oms/div)
(a)
{b)
(c)
Fig.5 Bridge type current limiter
(a) circuit topology; (b) CmTcnt waveform; (c) short circuit cmment limiting
Therefore the largest fault current occurred
during short circuit at the voltage of source vi,,
across zero, when to = 0. The peak value of the
fault current i m in this case is:
ips
coL + I~,
Here I,,, is the reactor current just before short
circuit, which is equal to the amplitude of steady
state load current.
Considering the current limiter topology of
the DC circuit in Fig. 3, the bridge circuit in
Fig. 5 can be further improved as shown in
Fig. 6. Here a switch device such as GTO can be
used as switch S and the diode Do is used as
freewheeling diode. In steady state, due to the
freewheeling diode, the bridge has no effect on
the load current. In fault condition the gate controlled switch can be switched off within tens of
microseconds. With such a short period the reactance of the limiting reactor L can be designed
much smaller than that of the topology in Fig. 5.
Also in Fig. 6, when the fault current is switched
off by the switch S, the current of reactor L is
freewheeled by diode Do. So the turning off of
the switch S is easily without transient resonance
activated by the limiting reactor.
~
Fig. 6
Load[
Fast interruption linfiter
44
WU Zhaolin, TAN Lingyun et al.
FURTHER IMPROVEMENT OF CURRENT LIMITER IN AC CIRCUIT
The topology of the current limiter in Fig. 5
is a haft-controlled bridge, in which the energy
of the reactor can only be charged in but can not
feed back to the source. If a full controlled
bridge is used as shown in Fig. 7, it can be operated as a rectifier or an inverter only depending
on the trigger angle of the controlled pulses.
Therefore, the energy stored in reactor L can
i ~ L
h ~
.
Vin
" J 'SLoad
Fig.7 Full controlled bridge type limiter
flow bilaterally. In normal operating condition,
with thyristors $2 and $3 fuctioning as freewheeling diodes with gate control pulses on them continuously, St and $4 are switched on and off alternately to keep load current flowing freely. So
the operating performance is the same as that in
Fig. 5. In case of short circuit, say in the positive haft cycle of source voltage at tl in Fig. 8 a,
the fault current rises quickly to the value of the
reactor current and the thyristor $3 is reversely
biased to be switched off. In the period ta - t2
the reactor is inserted in the fault circuit auto-
50A
i i [v,,-50V(rms),R~,,~a=10f2, L,m~,~,=I0mH
1!~
IOA f~_L. .
0
50A
t
IOA i 't
0
70V
0
7ov :. ill
o
matically with only St and $2 at on state. The
fault current then rises slowly L until t 2 . During
t2 - t3 the gate trigger pulses are controlled such
that the bridge operates as an inverter, so the
energy stored in the reactor at t 2 feeds back to
the voltage source. And the reactor current, as
well as the fault current, fails to zero to automatically turn off the conducting thyristors.
In some cases, if time delay of switching off
is required for coordination protection requirements, a special control strategy is developed.
In Fig. 8 ( a ) after t3, when the fault current
falls to zero, the bridge is controlled to operate
as a controllable rectifier with the trigger angle a
greater than 90 ~ . With only a reactor L at the
DC side, the steady state short circuit current
can be maintained as long as desired. In this
case the desired short circuit current can be adjusted as the trigger angle a is controllable. The
larger the a is, the smaller is the short circuit
current even if the reactance of L is the same. It
means that with the same desired current if a is
greater than 90 ~ a smaller L can be used.
When a is equal to 90 ~ the steady state fault
current reaches its greatest value. With such a
control strategy, multiple reclosing operation of
the fault circuit can easily be realized without
producing transients.
When
reclosing
is
required, the trigger angle a should be controlled such that initially it is very large, then it
is decreased smoothly until the thyristors are fully conducting.
Fig. 8 ( a ) is the simulation result of the current limiter operation. After interrupting the
short circuit current, a reclosing operation is
carried out successfully with the load current increasing smoothly to its normal value. Fig. 8 ( b )
.
.ilFy ~
t,t:t~
30A/di,
30A/di'~
.
~
i' A
00V/di~
'
100V/db
v,/vvvv~,
q:ime(50ms/div)"
(a)
[
(b)
Time(50ms/div)
(c)
Fig. 8 Short circuit protection
(a) simulation result; (b) experimental result of reclosing success; (c) experimental result of reclosing failure
Short circuit current timiter in AC network
is the e x p e r i m e n t a l result of the limiter, which
verifies the s i m u l a t i o n , as well as the theoretical
analysis results d i s c u s s e d a b o v e . Fig. 8 ( e )
shows the e x p e r i m e n t a l result of reelosing to a
p e r m a n e n t fault. W i t h the reclosing control strategy discussed a b o v e , reclosing of a fault circuit
with very large current impulse c a n b e fully
a v o i d e d . It m e a n s that soft reelosing strategy c a n
easily b e realized.
APPLICATIONS OF CURRENT LIMITER IN DISTRIBUTION POWER SYSTEMS
In some p a p e r s ( S l a d e et a l . , 1 9 9 2 ; Smith
et al. , 1 9 9 3 ) the a p p l i c a t i o n s of CLD ( c u r r e n t
limiting d e v i c e ) or C L I D ( c u r r e n t limiting interrupting device ) to p o w e r systems a n d their
r e q u i r e m e n t s are d i s c u s s e d in detail with the typical d i a g r a m of substation in Fig. 9 . T h e p r o p osed topology shown in Fig. 5 can b e a p p l i e d as
C L I D at the bus tie location " l " ( F i g . 9 ) to limit
the fault current through the bus to as low as d e sired a n d interrupt the fault current q u i c k l y within one h a l f cycle to m e e t all r e q u i r e m e n t s m e n tioned ( S l a d e et a l . , 1 9 9 2 ) . If very fast interruption is required the topology p r o p o s e d in Fig .
6 should b e a p p l i e d , with which the fault current
c a n b e interrupted within tens of m i c r o s e c o n d s .
In this c a s e with the limiting function of the rea c t o r a n d so short d u r a t i o n of interruption, the
fault current through the bus c a n not rise o b v i ously from that just before the short circuit.
T+~ T+
+
[] [ ]
Fig.9
-cI D oCllD
Diagram of distribution system
T h e control strategy of the current limiter topology p r o p o s e d in F i g . 7 is very flexible. T h i s
topology c a n b e u s e d as a CLID functioning the
s a m e way as that of the topology shown in Fig. 5
( a ) . F u r t h e r m o r e , as a CLD or C L I D , it c a n
45
also m a i n t a i n the limited fault c u r r e n t for as long
as desired to m e e t all r e q u i r e m e n t s for coordinating d o w n s t r e a m a n d u p s t r e a m protecting d e v i c e s ,
including reclosing r e q u i r e m e n t . Therefore it can
be u s e d as CLD or CLID for a p p l i c a t i o n to any
part of a distribution network ( F i g . 9 ) s u c h as
bus tie, f e e d e r , and m a i n s t r a n s f o r m e r .
CONCLUTIONS
More attention has b e e n p a i d in r e c e n t year
to the use of current limiting d e v i c e s for limiting
fault current in p o w e r n e t w o r k s . S o m e novel topologies of current limiter using p o w e r electronic
technology are p r e s e n t e d . T h e i r operating princip l e s , p e r f o r m a n c e s , a n d eontrol strategies are
d i s c u s s e d in detail. Simulations a n d e x p e r i m e n t s
on prototype m o d e l s were carried out to verify the
analysis r e s u l t s . T h e novel C L D s and C L I D s
p r o p o s e d in this p a p e r c a n m e e t all r e q u i r e m e n t s
c o n c l u d e d b y E P R I ( S l a d e et a l . , 1992; Smith
et a l . , 1 9 9 3 ) for CLD and C L I D a p p l i e d to distribution s y s t e m s at locations of b u s tie, feeders
and m a i n s t r a n s f o r m e r s .
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